1. Field of the Invention
The present invention relates to a reinforced polypropylene resin composition having only small protuberances of welds when molded, and a wheel cover molded from the reinforced polypropylene resin composition having an excellent appearance quality, heat resistance and mechanical properties such as impact strength, stiffness and modulus in flexure.
2. Description of Related Art
Glass-fiber-reinforced polypropylene resin compositions (hereinafter referred to as reinforced PPG) are widely used in industrial products such as automobile parts, electric appliance parts, and the like, because they generally provide excellent mechanical strength, stiffness and heat resistance at a low cost. Specific examples of the automobile parts are engine compartment parts, such as a battery tray, core material for an instrument panel, timing-belt cover, fan shroud, and the like. Recently, employment of reinforced PPG compositions in external trims such as fenders, rocker panels, wheel covers, and the like has been investigated.
Wheel covers are used, for example, for protecting clamp bolts of wheels and improving the appearance of wheels on an automobile. Japanese Utility Model Registration Application Kokai No. 1-62333 and Japanese Patent Application Kokai No. 3-137150 disclose a reinforced PPG used as a material for wheel covers because of its excellent mechanical strength, stiffness, heat resistance and low cost.
However, the most serious problem in the employment of a reinforced PPG in external trims, including wheel covers, is that a resin molded article made of the reinforced PPG has a poor surface appearance quality because it contains glass fiber.
Therefore, for improving the surface appearance, there has been proposed: the addition of an inorganic substance such as barium sulfate, calcium carbonate, talc or the like in Japanese Patent Application Kokoku No. 1-32856; the employment of the most suitable diameter of glass fiber and the most suitable glass fiber finishing agent in Japanese Patent Application Kokoku No. 4-12297; the addition of fibrous calcium silicate in Japanese Patent Application Kokoku No. 63-67498; the addition of ethylene-.alpha.-olefin rubber in Japanese Patent Application Kokoku No. 63-67499; and the employment of the most suitable diameter of glass fiber, the most suitable glass fiber binder and the most suitable pigment component in Japanese Patent Application Kokoku Nos. 64-6658, 64-7618 and 64-9340.
In the conventional methods, the surface appearance is improved by preventing the glass fiber from rising to the surface of the resin molded article, improvement of the surface gloss, and prevention of silver streaks which are silver lines formed by glass fiber. The conventional methods are only slightly effective in improving the surface appearance.
Employment of a conventional reinforced PPG composition, however, is disadvantageous in that protuberances are formed on the welds of a molded article made of the reinforced PPG composition.
In general, the molded article is often produced by injection molding. For injection molding, in some cases, a molten resin is, as shown in FIG. 1(A) (prior art), casted through a plurality of gates 91 into the cavity of a mold. In other cases, as shown in FIGS. 1(B) (prior art) and 1(C) (prior art), the molded article 9, the product, has a shape with an opening 92 and a boss 95. In such cases, a plurality of resin flows are formed in the cavity, so that in the molded article 9, a weld 93 is formed in a place in which the flows coverage upon each other. For example, in the production of a wheel cover, a plurality of resin flows are formed in the cavity of a molding tool, so that, as shown in FIG. 2 (prior art), welds 93 are formed in the wheel cover 9 in places in which the resin flows 90 coverage. In FIG. 2, numeral 91 shows the position of a gate during molding and numeral 91, an air hole or a hole for heat dissipation of the wheel cover.
As shown in FIG. 3 (prior art), the weld 93 forms protuberances 8. The protuberances 8 are particularly high when a reinforced polypropylene resin composition containing glass fiber 82 is used, which deteriorates the surface appearance of the molded article. The height L of the protuberances reaches 40 to 100 .mu.m using a conventional reinforced polypropylene resin composition.
In particular, because of the recent demand for improvement in decorative design and appearance, wheel covers are required to have various shapes, for example, the positions or shapes of the above-mentioned air holes are complicated or the air holes are enlarged. Therefore, the surface appearance at welds attracts considerable attention.
From the viewpoint of surface appearance, the height of the protuberances must be about 35 .mu.m or less, preferably 30 .mu.m or less.
For reducing the height of the protuberances, a method in which the length of glass fiber is reduced as much as possible (the average length is usually 300 .mu.m or less) by intense kneading in a production process of a PPG material, and a method in which the amount of glass fiber added is reduced are known. These methods, however, are disadvantageous in that with a decrease of the height of the protuberances of welds, the mechanical properties such as impact strength, stiffness and modulus in flexure, and the heat resistance are deteriorated.